Waterhammer
is an impact load that is the most misunderstood force known to
pressure transducers today. A waterhammer is created by stopping
and/or starting a liquid flow suddenly. The results of a waterhammer
or impulse load are devastating to a pressure sensor. The impulse
load occurs suddenly, in the millisecond time frame, but the effects
of it last a life time. Waterhammers occur in almost all pressure
systems and usually can not be stopped without extensive time,
energy and studies.

A
common example of a waterhammer occurs in most homes everyday.
Simply turning off a shower quickly sends a loud thud through
the house; this is a perfect example of a waterhammer. Dishwashers
and washing machines make these same sounds, because inside them
small solenoid valves are being opened and closed quickly, producing
this pulse noise. The key phrase in the examples above was turning
on or off the water "quickly" verses turning it off slowly. In
the shower example, if you turn the water off slowly, the waterhammer
will not occur. Common industrial hardware like relief valves,
solenoid valves, valves in general, centrifugal pumps, positive
displacement pumps, and regulators can and will cause heavy hammer
effects. A simple solution to this devastating effect is to protect
each sensor with a pressure snubber. Snubbers are low ticket items
that will insure that this hammer effect will not render your
costly sensor useless. All pressure sensors should utilize snubbers
for all installations.

The hammer occurs because an entire train of water is being stopped
so fast that the end of the train hits up against the front end
and sends shock waves through the pipe. This is similar to a real
train, instead of slowing to a stop, it hits into a mountain side.
The back of the train continues forward even though the front
can not go anywhere. Since the water flow is restricted inside
the pipe, a shock wave of incompressible water travels back down
the pipe deflecting everything in its path. An unprotected transducer
in the path of this monstrous wake is without question, going
to sustain heavy damage.

To
understand the damage caused by the waterhammer forces, it is
necessary to understand the principles behind the sensor. Most
pressure sensors utilize a rigid diaphragm as the primary sensing
element. The diaphragm deflects due to the pressure, and its deflection
is transformed to an electrical output via various methods. The
key component is the rigid diaphragm. The rigid diaphragm deflects
only on the order of a thousandth of an inch. With a large wake
of fluid hitting the sensor, it is no wonder the diaphragm is
bent beyond its elastic limit and permanent damage is done. Remember
that a snubber eliminates this effect and therefore should always
be installed on every pressure system.

Snubbers
are chosen by the media that they will be used on such as liquids,
gases or dense liquids like motor oils, and their physical mounting
fittings. Snubbers only let so much fluid pass through per unit
time, eliminating the surge from hitting the diaphragm. Liquids
possess a large hammer effect because they are incompressible,
but gases can also possess a hammer effect large enough to render
a sensor useless. A practical analogue to a snubber is a sponge
in the drain of a sink. The sponge ensures that the sink empties
slowly, instead of all at one. A lot of common questions are asked
about hammer effects; the following are just a few.

WILL
A SNUBBER EFFECT THE RESPONSE TIME OF MY PRESSURE TRANSDUCER?

In
most cases, the transducer is connected to a meter of a recorder
that updates at 2 to 3 times a second; therefore a snubber will
not effect it at all.

WHAT
ARE THE SYMPTOMS THAT MY SENSOR HAS BEEN DAMAGED BY A FLUID HAMMER?

Most sensors will exhibit a higher than normal output at zero
pressure (a zero shift). This occurs because the diaphragm can
not return to zero. In severe cases no output occurs or the output
does not change with an increase in pressure.

IF
MY SENSOR HAS A LARGE ZERO OFFSET CAUSED BY THIS HAMMER EFFECT
CAN IT BE REPAIRED?

Most sensors are non-repairable. The diaphragm is the main building
block of the sensor. When building a sensor the diaphragm is first
built and the all the other components are chosen to achieve the
rated specification. When a diaphragm bends beyond its elastic
limit, it can not be bent back to original shape or replaced because
of the unique components associated with the original diaphragm.
If a diaphragm does have a slight zero shift, less than 10%, it
probably is still linear and can be used. Before reinstalling
it in the system, please acquire a snubber or the hammer effect
will occur again and possibly damage the unit further.

WILL
A SNUBBER STOP AN OVERPRESSURE?

Snubbers
stop spikes only, they do not perform miracles. An overpressure
will not be stopped a snubber. A spike lasts only on the order
of milliseconds; any overpressure for more than that time will
damage the sensor.

HOW
IS A SNUBBER INSTALLED IN A PRESSURE SYSTEM?

The
snubber would screw on to the front end of the transducer and
then thread into the piping system. The snubber is located between
the piping under pressure and the pressure transducer. The following
brief equations summarize the hammer effect and is followed by
an example of waterhammers destructive forces. The following equation
determines the maximum pressure change that occurs during a fluid
hammer. The equation assumes that the piping is inelastic.

WHERE

P
is the change in pressure resulting from the fluid hammer (pounds
per square foot)

r
is the fluid density (pound mass per cubic foot)

c
is the speed of sound in the fluid (feet per second) v is the
change in velocity of the fluid (feet per second)

g
is the gravitational constant (32.2 feet per second per second)

E
is the bulk modulus of the fluid media (listed in PSI but must
be converted to PSF)

In
this example, a 1 inch pipe with a flow rate of 10 gpm had a hammer
effect resulting in an increase in pressure of 243 PSI above normal
operating conditions. Considering normal city water pressure of
50 PSI, most end users would select a sensor of approximately
100 PSI full scale to be on the safe side. A 100 PSI sensor usually
has an over pressure of 200% associated with it, meaning it will
be able to withstand 200 PSI. Now the hammer increases the system
from 50 PSI to 293 PSI (50 + 243), which is overpressurizing the
transducer and causing damage to it. Most end users are puzzled
as to how a system that is supplied with only 60 PSI is capable
of producing over 200 PSI. After reading this article it should
be evident that fluid hammers are a complex phenomena with a simple
solution: installing snubbers on all pressure transducers.